Ruthenium hydride complexes of chiral and achiral diphosphazane ligands and asymmetric transfer hydrogenation reactions

Article abstract of DOI:10.1016/j.jorganchem.2006.12.042

The half-sandwhich ruthenium chloro complexes bearing chelated diphosphazane ligands, [(η⁵-Cp)RuCl{κ²-P,P-(RO)₂PN(M e)P(OR)₂}] [R = C₆H₃Me₂-2,6] (1) and [(η⁵-Cp^*<

Full text of DOI:10.1016/j.jorganchem.2006.12.042

Journal of Organometallic Chemistry 692 (2007) 1875–1891
www.elsevier.com/locate/jorganchem
Ruthenium hydride complexes of chiral and achiral diphosphazane
q
ligands and asymmetric transfer hydrogenation reactions
Thengarai S. Venkatakrishnan, Swadhin K. Mandal, Raghuraman Kannan,
*
Setharampattu S. Krishnamurthy , Munirathinam Nethaji
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
Received 1 December 2006; received in revised form 27 December 2006; accepted 27 December 2006
Available online 18 January 2007
Dedicated to our colleague M. Raja (Late), whose sudden demise had snatched away from our midst a promising young researcher.
Abstract
The half-sandwhich ruthenium chloro complexes bearing chelated diphosphazane ligands, [(g5-Cp)RuCl{j2-P,P-(RO)2PN(Me)-
P(OR)2}] [R = C6H3Me2-2,6] (1) and [(g5-Cp*)RuCl{j2-P,P-X2PN(R)PYY0}] [R = Me, X = Y = Y0 = OC6H5 (2); R = CHMe2,
t
X2 = C20H12O2, Y = Y0 = OC6H5 (3) or OC6H4 Bu-4 (4)] have been prepared by the reaction of CpRu(PPh3)2Cl with (RO)2PN(Me)-
P(OR)2 [R = C6H3Me2-2,6 (L1)] or by the reaction of [Cp*RuCl2]n with X2PN(R)PYY0 in the presence of zinc dust. Among the four
diastereomers (two enantiomeric pairs) possible for the ‘‘chiral at metal’’ complexes 3 and 4, only two diastereomers (one enantiomeric
pair) are formed in these reactions. The complexes 1, 2, 4 and [(g5-Cp)RuCl{j2-P,P-Ph2PN((S)-*CHMePh)PPhY}] [Y = Ph (5) or
N2C3HMe2-3,5 (SCSPRRu)-(6)] react with NaOMe to give the corresponding hydride complexes [(g5-Cp)RuH{j2-P,P-(RO)2PN(Me)-
t
P(OR)2}] (7), [(g5-Cp*)RuH{j2-P,P0-X2PN(R)PY2}] [R = Me, X = Y = OC6H5 (8); R = CHMe2, X2 = C20H12O2, Y = OC6H4 Bu-4
(9)] and [(g5-Cp)RuH{j2-P,P-Ph2PN((S)-*CHMePh)PPhY}][Y = Ph (10) or N2C3HMe2-3,5 (SCSPRRu)-(11a) and (SCSPSRu)-(11b)].
Only one enantiomeric pair of the hydride 9 is obtained from the chloro precursor 4 that bears sterically bulky substituents at the phos-
phorus centers. On the other hand, the optically pure trichiral complex 6 that bears sterically less bulky substituents at the phosphorus
gives a mixture of two diastereomers (11a and 11b). Protonation of complex 7 using different acids (HX) gives a mixture of [(g5-
Cp)Ru(g2-H2){j2-P,P-(RO)2PN(Me)P(OR)2}]X (12a) and [(g5-Cp)Ru(H)2{j2-P,P-(RO)2PN(Me)P(OR)2}]X (12b) of which 12a is the
major product independent of the acid used; the dihydrogen nature of 12a is established by T1 measurements and also by synthesizing
the deuteride analogue 7-D followed by protonation to obtain the D–H isotopomer. Preliminary investigations on asymmetric transfer
hydrogenation of 2-acetonaphthone in the presence of a series of chiral diphosphazane ligands show that diphosphazanes in which the
phosphorus centers are strong p-acceptor in character and bear sterically bulky substituents impart moderate levels of enantioselectivity.
Attempts to identify the hydride intermediate involved in the asymmetric transfer hydrogenation by a model reaction suggests that a
complex of the type, [Ru(H)(Cl){j2-P,P-X2PN(R)PY2}(solvent)2] could be the active species in this transformation.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Diphosphazanes; Half-sandwich complexes; Ruthenium; Hydride complexes; Asymmetric transfer hydrogenation; Catalysis
1. Introduction
The characterization and isolation of coordinatively and
electronically unsaturated, highly reactive and therefore
short-lived transition metal complexes is an area of consid-
q
Part 26 of the series ‘‘Organometallic chemistry of diphosphazanes’’;
for Part 25, see [12d].
erable interest owing to their importance as key intermedi-
*
Corresponding author. Tel.: +91 80 22932401. Fax: +91 80 23600683/
ates in various homogeneous catalytic processes such as
hydrogenation, hydrosilylation, hydroformylation and
23601552.
E-mail address: sskrish@ipc.iisc.ernet.in (S.S. Krishnamurthy).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.12.042

1876
T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
Table 1
transfer-hydrogenation reactions [1,2]. Among the various
catalytic transformations, ruthenium catalyzed asymmetric
transfer hydrogenation has evoked considerable interest
[2]. Experimental and theoretical studies reveal that the
reaction proceeds via the formation of a catalytically active
metal–hydride species [3–5]. Several research groups have
been working on the design of novel ligands for the ruthe-
nium catalyzed transfer hydrogenation of ketones and the
chiral variations thereof to achieve high levels of enantiose-
lectivity. A range of transition metal complexes bearing
homo- and hetero-donor P–P [6], N–N [6a,7], P–N [8],
N–O [9] ligands, terdentate ligands [10] and mixed homo-
donor ligands [11] have been reported and their efficiency
in transfer hydrogenation investigated. Since the active cat-
alyst or pre-catalyst involves a ruthenium hydride species,
the continuing interest could be attributed to the quest to
characterize and isolate the hydride intermediates involved
in these reactions by fine-tuning the steric and electronic
properties of the ancillary ligands. The catalytic species is
usually generated in situ in the reaction medium from a
stable Ru(II) species such as [RuCl2(PPh3)3] or
[RuCl2(DMSO)4] and the chiral auxiliary.
Recently we reported the synthesis of cyclopentadienyl
ruthenium complexes of chiral and achiral diphosphazane
ligands [12a]. In continuation of our interest [12a–d] in
the organometallic chemistry of diphosphazane ligands
[13], we report herein the ruthenium(II) hydride complexes
of chiral and achiral diphosphazanes and preliminary
investigations on ruthenium catalyzed asymmetric transfer
hydrogenation of 2-acetonaphthone in the presence of chi-
ral diphosphazanes.
Yields, melting points and CHN analysis for the compounds synthesized
in the present study
Compound
Yield
MP
Elemental analysisa
C (%)
H (%)
N (%)
L1
1
61
25
30
30
35
45
70
60
152 (d)
182 (d)
178 (d)
180 (d)
182 (d)
175 (d)
–
130 (d)
190 (d)
180 (d)
184 (d)
69.6 (68.8)
58.4 (58.7)
57.1 (57.2)
62.8 (62.7)
65.5 (65.6)
61.2 (61.4)
67.8 (67.7)
68.6 (68.8)
57.0 (57.1)
59.6 (59.9)
64.2 (64.3)
7.2 (6.8)
5.9 (5.7)
5.2 (5.2)
5.0 (5.1)
6.3 (6.3)
6.0 (6.1)
5.3 (5.4)
5.2 (5.3)
5.2 (5.2)
4.9 (5.0)
4.3 (4.4)
3.2 (2.4)
2.4 (1.8)
2.1 (1.9)
1.8 (1.6)
1.4 (1.3)
2.1 (1.9)
2.2 (2.1)
2.4 (2.5)
4.5 (4.7)
1.7 (2.1)
1.5 (1.8)
2
3
4
7
10
14
18
19
20
a
<10%
80
80
Calculated values are in parentheses.
nyl), [RuCl2(PPh3)3] [15b] and [RuCl2(COD)]n [15c] were
prepared according to literature methods. MeN(PCl2)2
was prepared according to Nixon’s procedure [16].
[Cp*RuCl2]n (Cp* = pentamethyl cyclopentadienyl), CF3-
SO3H, HBF4 Æ Et2O, CD3OD, 2,20-bipyridine, CF3SO3Ag,
P(OPh)3 (Aldrich), 2,6-dimethyl phenol (Merck) and
P(OMe)3 (local source) were used as received.
The yields, melting points and elemental analyses for the
compounds synthesized in the present study are listed in
Table 1. The spectroscopic data for the compounds and
the results of HPLC studies are presented in Tables 2–4.
2.2. Synthesis of (RO)2PN(Me)P(OR)2 [R = C6H3Me2-
2,6] (L1) [17]
A benzene (60 cm3) solution of Cl2PN(Me)PCl2 (2.33 g,
0.01 mol) was added dropwise to a benzene (60 cm3) solu-
tion of 2,6-dimethylphenol (4.88 g, 0.04 mol) and triethyl-
amine (8.4 cm3, 0.06 mol) at 0 ꢁC over a period of
15 min. The reaction mixture was stirred at 25 ꢁC for
24 h and heated under reflux for 8 h. The precipitate of
Et3N Æ HCl was filtered and solvent evaporated from the fil-
trate in vacuo to obtain a light yellow oily residue which
was purified by column chromatography using benzene/
petrol (1:1 v/v). Evaporation of the eluant yielded the title
compound as a colorless oil. The oil was dissolved in
dichloromethane–petrol and the solution cooled to 0 ꢁC
to obtain a colorless solid. Traces of unreacted phenol
adhering to the solid could be removed by triturating the
product with petrol. Single crystals of the compound suit-
able for X-ray diffraction were obtained by slow evapora-
tion of a toluene solution of the compound.
2. Experimental
2.1. General
All reactions and manipulations were carried out under
an atmosphere of dry nitrogen using standard Schlenk and
vacuum-line techniques. The solvents were purified by
standard procedures and distilled under nitrogen prior to
use. The NMR spectra (1H and 31P{1H}) were recorded
in CDCl3 at 298 K using Bruker ACF-200, Bruker AMX-
400 or Bruker Avance-400 spectrometers. Elemental
analyses were carried out using a Perkin–Elmer 2400
CHN analyser. Melting points were recorded in a Buchi
B-540 melting point apparatus and were uncorrected.
The diphosphazane ligands X2PN(R)PY2 [R = Me,
X = Y = OC6H5 (L2) [14a]; R = CHMe2, X2 = C20H12O2,
t
Y = OC6H5 (L3) or OC6H4 Bu-4 (L4) [12b]; X = Y = Ph,
R = (S)-*CHMePh(L5) [14b] or CHMe2 (L6) [14c];
R = Me, X2 = Y2 = (R) or (S)-C20H12O2 (L7) [14d] and
R = CHMe2, X2 = (R) or (S)-C20H12O2, Y = C6H5 (L8)
2.3. Synthesis of [(g5-Cp)RuCl{j2-P,P-
(RO)2PN(Me)P(OR)2}] [R = C6H3Me2-2,6] (1)
[14e];
(SCRP)-Ph2PN((S)-*CHMePh)P(Ph)(N2C3HMe2-
A 100 cm3 double necked flask was charged with a mix-
ture of (g5-Cp)Ru(PPh3)2Cl (0.200 g, 0.203 mmol) and the
ligand L1 (0.117 g, 0.203 mmol) under nitrogen atmosphere.
The mixture was dissolved in benzene (50 cm3) and the solu-
3,5) (L9) [14b], the complexes [(g5-Cp)Ru0Cl{j2-P,P-
X2PN((S)-*CHMePh)PYY0}] [X = Y = Ph, Y = Ph (5)
or N2C3HMe2-3,5 (SCSPRRu)-(6)] [12a] and the starting
materials [(g5-Cp)Ru(PPh3)2Cl] [15a] (Cp = cyclopentadie-

T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
1877
Table 2
NMRa (1H and 31P) data for ligand L1 and half-sandwich ruthenium complexes 1–13
31P{1H}
1H
db
Ddc
PA
–
d
PA
PX
–
PX
–
Cp/Cp*
–
CH3
Ru–Hd
–
L1
1
135.1 (s)
3.51 (t)e
2.13 (s)f
108.8 (s)
–
ꢀ26.3
–
4.25
3.39 (t)e, 2.34,
2.17 (s)f
–
2
3
4
99.1 (s)
123.8 (d 60.8)
124.8 (d 60.8)
–
ꢀ36.4
ꢀ32.6
ꢀ33.4
–
1.78
2.11
1.36
2.44 (t)e
–
–
–
107.7 (d)
106.8 (d)
ꢀ28.5
1.28, 0.93 (d)g
1.18, 0.80 (d)g,
1.35, 1.33 (s)h
3.13 (t)e,
ꢀ30.0
7
120.6 (s)
–
–
ꢀ14.5
ꢀ14.4
–
–
4.13
4.13
ꢀ13.06 (t 34)
2.20, 2.17 (s)f
3.13 (t)e,
2.20, 2.17 (s)f
2.68 (t)e
7-D
120.7 (t JD–P = 5.0)
–
8i
9
114.1 (s)
122.5 (d 64.8)
–
ꢀ21.4
ꢀ35.1
–
3.37
2.64
ꢀ11.99 (t 34)
ꢀ11.39 (t 35)
118.3 (d)
ꢀ19.0
1.48, 0.90 (d)g,
1.32, 1.28 (s)h
0.90 (d)g
10j
100.4 (d 107.0)
147.2 (d 107.0)k
137.4 (d 99.0)k
115.5 (s)
107.8 (s)
115.6 (t JD–P = 13.8)
108.8 (br)
100.9 (br)
111.9 (d)l
101.9 (d)l
–
+48.2
+48.7
4.57
4.69
ꢀ10.70 (t 30)
11a,bi,j
1.92 (d)g,
ꢀ11.06 (dd 34, 26),
ꢀ11.86 (dd 34, 29)
ꢀ7.93 (br),
3.00, 2.57 (s)m
3.35 (t)e,
12a
12b
13a
13b
a
ꢀ19.6
ꢀ27.3
ꢀ19.5
ꢀ27.1
–
–
4.65
4.71
4.65
4.71
2.20, 2.16 (s)f
3.35 (t)e
ꢀ6.14 (t 27.4)
–
ꢀ8.00 (t JD–H = 24.5),
2.20, 2.16 (s)f
ꢀ6.20 (tt 26.4, JD–H = 4.0)
Recorded in CDCl3 in a 200 MHz or 400 MHz spectrometer.
2J(P,P) values in parentheses.
b
c
d(complex) ꢀ d(free ligand).
d
e
2J(P,H) coupling constants in parentheses.
CH3, N(CH3), 3J(P,H) = 7.0 ꢀ 12.0 Hz.
CH3, OC6H3Me2-2,6.
f
g
h
i
CH3 protons of (S)-*CHMePh or CHMe2 group are doublets with 3J(H,H) = 7.0 Hz.
tBu, OC6H4 Bu-4.
t
1H NMR recorded in C6D6 and 31P NMR in benzene (200 MHz).
AB spin system.
j
k
l
PPh(N2C3HMe2-3,5) phosphorus.
PPh2 phosphorus.
3,5-Dimethyl pyrazole group protons.
m
residue. The residue was dissolved in ethanol (8–10 cm3)
and kept at ambient temperature for 5 days. The title com-
pound precipitated as a bright yellow-orange solid.
Table 3
Results of catalytic asymmetric transfer hydrogenation of 2-
acetonaphthonea
Ligand
% Yield
% ee
Enantiomer
(S)-L3
(R)-L4
(S)-L4
L5
87.7
16.3
13.0
85.9
68.5
69.2
85.2
85.0
88.5
>99
rac
35
34
rac
10
12
rac
rac
rac
30
–
2.4. Synthesis of [(g5-Cp*)RuCl{j2-P,P-X2PN(R)PYY0}]
[R = Me, X = Y = Y0 = OC6H5 (2); R = CHMe2,
R
S
–
R
S
t
X2 = C20H12O2, Y = Y0 = OC6H5 (3) or OC6H4 Bu-4 (4)]
(R,R)-L7
(S,S)-L7
(R)-L8
(S)-L8
L9
A 50 cm3 double necked flask was charged with a mix-
ture of [(g5-Cp*)RuCl2]n (0.050 g, 0.163 mmol), the corre-
sponding diphosphazane (0.179 mmol) and zinc dust
(0.053 g, 0.815 mmol) under nitrogen atmosphere. Toluene
(30 cm3) was added and the reaction mixture stirred for 24 h
during which the dark brown solution turned bright yellow
orange. The 31P NMR spectrum of the reaction mixture dis-
played resonances corresponding to unreacted diphosphaz-
ane in addition to those arising from the product. Heating
the reaction mixture under reflux did not improve the yield
of the product. The solution was filtered and the solvent
evaporated to dryness. The residue was dissolved in ethanol
–
–
R
(R,R)-L7,b
a
200:1:2 ratio of ketone:Ru:ligand.
100:1:2 ratio of ketone:Ru:ligand.
b
tion heated under reflux for 60 h. The reaction mixture was
concentrated to 5–10 cm3 and kept at ambient temperature
during which a black mass separated out. The solution was
filtered and the solvent evaporated to dryness to obtain a

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T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
Table 4
NMRa (1H and 31P) data for ruthenium hydride complexes 14–20
31P{1H}b
1H
d
d
Ddc
CHd
CH3
Ru–Hf
e
PA
PX
PA
PX
14g
15
91.9
87.5 (s)
114.3 (s)
84.9
–
–
+39.1
+38.7
ꢀ20.8
+32.1
–
–
4.66
4.01
–
1.08 (d)
ꢀ15.27 (q 20)
ꢀ15.17 (q 20)
ꢀ14.88 (t 33.3)
0.60 (d)
16
3.23 (t)h,
2.13 (s)i
17j
18j
19
108.9(d 36.7)
97.6 (d 9.7)
107.7 (d)
91.8 (d)
89.5 (m)
88.7(d)
ꢀ26.2
ꢀ27.4
ꢀ43.3
+37.3
+36.5
–
3.32 (t)h,
2.11 (s)i
ꢀ13.39 (dd 44.0, 33.3)
–
ꢀ37.5
–
3.74 (t)h,
2.51, 2.65 (s)i
2.17 (d)k,
0.18 (d)
129.4 (q 31.9)
118.1 (q 32.4)
–
–
4.40
4.45
ꢀ6.10 (dq 120, 20)
ꢀ6.25 (dq 140, 20)
20
a
b
c
0.24 (d)
Recorded in CDCl3 in a 200 MHz or 400 MHz spectrometer except for complex 16 (recorded in CD3CN).
2J(P,P) coupling constants are given in parentheses.
Dd = d(complex) ꢀ d(free ligand).
d
e
CH proton of (S)-*CHMePh or CHMe2 group are multiplets.
CH3 protons of (S)-*CHMePh or CHMe2 group are doublets with 3J(H,H) = 7.0 Hz.
Numbers within parenthesis are 2J(P,H) coupling constants.
AA0XX0 spin system.
f
g
h
i
CH3 protons of N(CH3) group, 3J(P,H) = 7.0 ꢀ 11.0 Hz.
CH3, OC6H3Me2-2,6.
j
31P NMR spectrum is an AB spin system.
k
Me, P(OMe)3, 3J(P,H) = 10.0 Hz.
(8–10 cm3) and subjected to column chromatography over
silica gel using the same solvent. Evaporation of the eluant
afforded the title compounds as bright orange solids. Single
crystals of complex 4 suitable for X-ray diffraction were
obtained from an ethanolic solution.
gas. The mixture was heated overnight under reflux and the
solution cooled. Solvent was evaporated to dryness to obtain
a pale yellow residue. Attempts to obtain a pure sample from
the residue were unsuccessful. Both complexes 8 and 9 were
characterized only by NMR spectroscopy.
2.5. Synthesis of [(g5-Cp)RuH{j2-P,P-
(RO)2PN(Me)P(OR)2}] [OR = OC6H3Me2-2,6 (7)]
2.7. Synthesis of [(g5-Cp)RuH{j2-P,P-Ph2PN((S)-
*CHMePh)PPhY}] [Y = Ph (10) or N2C3HMe2-3,5
(11a,b)]
The complex [(g5-Cp)RuCl{j2-P,P-X2PN(R)PX2}]
(0.009 g, 0.011 mmol) [X = OC6H3Me2-2,6 (1)] was added
to a solution of CH3ONa/CH3OH [prepared in situ by
the addition of Na (0.005 g, 0.220 mmol) to CH3OH
(3 cm3)] under a stream of nitrogen gas. The mixture was
heated under reflux for 20 h; methanol was added periodi-
cally to maintain the volume. The reaction mixture was
cooled to ambient temperature. Solvent was evaporated
to dryness to obtain a pale yellow residue of [(g5-
Cp)RuH{j2-P,P-(RO)2PN(R)P(OR)2}] (7). Single crystals
of the title compound were obtained by layering a toluene
solution of the compound with petrol.
A solid sample of the complex [(g5-Cp)RuCl{j2-P,P-
X2PN((S)-*CHMePh)PYY0}] (0.05 g, 0.07 mmol) [X = Y
= Ph, Y0 = Ph (5) or N2C3HMe2-3,5 (SCSPRRu)-(6)] was
treated with CH3ONa/CH3OH as described in Section
2.6. The mixture was heated under reflux for 3 h for 5 or
6 h for 6 and the solution cooled. A yellow solid precipi-
tated, which was filtered off and washed with pentane. Com-
plex 10 could be isolated in a pure form. Complex 11 was
found to be a diastereomeric mixture of 11a and 11b. Com-
plex 11a could be isolated in low yield (<10%) by dissolu-
tion of the yellow solid in deoxygenated pentane followed
by cooling the solution to 0 ꢁC for 2 h. The other diastereo-
mer 11b could not be separated in a pure form. Both 11a
and 11b were characterized only by NMR spectroscopy.
2.6. Synthesis of [(g5-Cp*)RuH{j2-P,P-X2PN(R)PYY0}]
[R = Me, X = Y = Y0 = OC6H5 (8); R = CHMe2,
t
X2 = C20H12O2, Y = Y0 = OC6H4 Bu-4 (9)]
2.8. Protonation of [(g5-Cp)RuH{j2-P,P-
(RO)2PN(Me)P(OR)2}] (7) using different acids, HX
(X = CF3SO3, BF4 or CF3COO)
A solid sample of the complex [(g5-Cp*)RuCl{j2-P,
P-X2PN(R)PYY0}] (0.011 mmol) [R = Me, X = Y = Y0 =
OC6H5 (2), R = CHMe2, X2 = C20H12O2, Y = Y0 =
t
OC6H4 Bu-4 (4)] was added to a solution of CH3ONa/
The protonation experiments were carried out in a
NMR tube sealed with a rubber septum. In a typical exper-
iment, complex 7 (0.005 g, 0.007 mmol) was dissolved in
CH3OH [prepared in situ by the addition of Na (0.005 g,
0.220 mmol) to CH3OH (3 cm3)] under a stream of nitrogen

T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
1879
CDCl3 (0.4 cm3) and HX (0.140 mmol) added; the mixture
was shaken well and subjected to NMR measurements. Use
of a stoichiometric quantity of the acid did not result in
protonation (no dihydride/dihydrogen resonances were
observed).
obtain a creamy yellow precipitate. The precipitate was
found to be stable in acetonitrile and sensitive towards hal-
ogenated solvents. The NMR spectrum of the precipitate
was recorded in CD3CN to obtain the spectroscopic data
for complex 16-d3. The complex could not be isolated in
pure form owing to its high sensitivity and was character-
ized only by spectroscopic data.
2.9. Transfer hydrogenation of 2-acetonaphthone
A 20 cm3 double necked flask fitted with a reflux con-
denser and an inlet for nitrogen was charged with
[RuCl2(PPh3)3] (0.014 g, 0.015 mmol) and chiral diphosp-
hazane (0.030 mmol). Freshly distilled and degassed (two
freeze–pump–thaw cycles) 2-propanol (6 cm3) was added
and the mixture heated to 80 ꢁC for 30 min. The mixture
was cooled to 25 ꢁC and 2-acetonaphthone (0.511 g,
3.000 mmol) and finely crushed KOH (0.002 g, 0.030 mmol)
were added. The mixture was then heated to 80 ꢁC for 16 h.
Solvent was evaporated from the reaction mixture to obtain
a dark brown residue. The yield of the product was calcu-
lated from the relative integrated intensities of the starting
material and the product as observed by the 1H NMR spec-
trum of the reaction mixture. The dark residue was dis-
solved in ethyl acetate-petrol (1:9 v/v) (ꢁ5–6 cm3) and
loaded on top of a silica gel column (20 cm · 1.5 cm) and
eluted with the same solvent (ꢁ250–300 cm3) during which
the starting material (2-acetonaphthone) was eluted. Fur-
ther elution gave the desired 2-(2-naphthyl)-ethanol, which
was obtained as a solid after evaporation of the eluant. The
enantiomeric excess (% ee) of the product was determined
by HPLC analysis (see below).
2.12. Synthesis of cis-[Ru(H)(Cl)(j2-P,P-L1)(j2-N,N-
2,20-bipyridine)] (17)
The creamy yellow precipitate obtained from the reac-
tion mixture as described above was dissolved in a mixture
of acetonitrile (15 cm3) and THF (6 cm3) to obtain the
complex trans-[RuH(Cl){j2-P,P-(RO)2PN(Me)P(OR)2}-
(CH3CN)2] (16). To this solution 2,20-bipyridine (0.028 g,
0.177 mmol) was added. The solution turned dark immedi-
ately. The solution was stirred at ambient temperature for
75 min. Solvent was evaporated from the reaction mixture
to obtain complex 17 as a dark brown solid. Attempts to
obtain a pure sample of complex 17 were unsuccessful.
Crystallization from dichloromethane–petrol afforded
the dichloro complex cis-[RuCl2{j2-P,P-(RO)2PN(Me)-
P(OR)2}(j2-N,N-2,20-bpy)] (18) as a bright yellow solid.
2.13. Synthesis of trans-[Ru(H){P(OR)3}{j2-P,P-
Ph2PN((S)-*CHMePh)PPh2}2] [R = Me (19) or Ph
(20)]
P(OR)3 (0.072 mmol) and AgOTf (0.018 g, 0.072 mmol)
were added to a THF (10 cm3) solution of complex 14
(0.080 g, 0.072 mmol), under nitrogen atmosphere and
the reaction mixture was stirred under dark at ambient
temperature for 1.5 h. The reaction mixture was filtered,
solvent evaporated under vacuo, washed with petrol and
dried to obtain a residue. The residue was crystallized from
dichloromethane solution layered with petrol to obtain the
title compounds as colorless solids.
2.10. Synthesis of trans-[Ru(H)Cl{j2-P,P-
Ph2PN(R)PPh2}2] [R = (S)-*CHMePh (14) or CHMe2
(15)]
A mixture of [RuCl2(cod)]n (0.050 g, 0.177 mmol),
diphosphazane (0.173 g for R = (S)-*CHMePh, 0.151 g
for R = CHMe2, 0.353 mmol) and Et3N (1 cm3) were sus-
pended in ethanol (30 cm3). The reaction mixture was
heated under reflux for 5 h and cooled to room tempera-
ture. The title compounds precipitated as bright yellow sol-
ids. The mother liquor was decanted; the solid was washed
with cold n-pentane (3 · 5 cm3) and dried in vacuo. Com-
plex 15 could not be isolated in pure form owing to its high
sensitivity to air and moisture. Both the complexes decom-
posed upon standing in halogenated solvents.
2.14. HPLC studies
A Merck-Hitachi HPLC system fitted with a binary high
pressure pump, Chiralcel-OD column, 20 ll injection loop
and UV detector was used to determine the enantiomeric
excess obtained in catalytic reactions. A mixture of n-hex-
ane and 2-propanol (95:5 v/v) at a flow rate of 0.5 cm3/
min was used as the eluant; the absorbance was monitored
at 254 nm. The absolute configuration of the product was
determined by comparison of the retention time in HPLC
profiles and also from optical rotation values reported in
the literature [9b].
2.11. Synthesis of trans-[Ru(H)(Cl)(j2-P,P-L1)(S)2]
[(S = CD3CN) (16-d3)]
Freshly distilled Et3N (1 cm3) was added to a mixture of
[RuCl2(COD)]n (0.050 g, 0.177 mmol) and L1 (0.102 g,
0.177 mmol). To the suspension, dry ethanol (30 cm3) was
added and the mixture heated under reflux for 5 h. A bright
yellow precipitate formed during the course of the reaction.
The reaction mixture was cooled to ambient temperature
and the alcohol layer was removed using a syringe to
2.15. X-ray crystallography
The crystals were mounted on a glass fiber and the
intensity data for all the compounds were obtained at room
temperature from
a Bruker SMART APEX CCD

1880
T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
diffractometer equipped with fine focus 1.75 kW sealed
tube Mo-Ka X-ray source with increasing x (width of
0.3ꢁ per frame) at an exposure time of n s/frame (n = 4
for L1, n = 8 for 4 and n = 25 for 18). The SMART [18a] soft-
ware was used for cell-refinement and data acquisition and
the SAINT [18b] software was used for data reduction. Lor-
entz and polarization corrections were made on the inten-
sity data. An absorption correction was made on the
intensity data using the SADABS [18c] program. The struc-
tures were solved using SHELXTL [18d] and the WinGX
graphical user interface [19]. Least-square refinements were
performed by the full-matrix method with SHELXL-97 [20].
Pertinent crystallographic data are summarized in Table
5. All non-hydrogen atoms were refined anisotropically
and hydrogen atoms were refined by allowing them to ride
with the respective non-H atoms. The solvent molecule
3. Results and discussion
3.1. Synthesis of a sterically bulky diphosphazane, L1
The sterically bulky diphosphazane (RO)2PN(Me)-
P(OR)2 (R = C6H3Me2-2,6) (L1) was prepared previously
in our laboratory [17]. The experimental details, spectro-
scopic data and crystal structure are reported here. The
reaction of Cl2PN(Me)PCl2 [16] with four equivalents of
2,6-dimethyl phenol in the presence of triethyl amine as
base affords the diphosphazane (RO)2PN(Me)P(OR)2
(R = C6H3Me2-2,6) (L1). The 31P NMR spectrum of the
compound displays a singlet at 135.1 ppm for the magnet-
ically equivalent phosphorus nuclei. A triplet is observed
for the methyl protons bound to the nitrogen; a singlet is
observed for the methyl group protons present at the ortho
positions of the phenoxy rings.
t
(ethanol) and the carbon atoms of the Bu groups [C(28),
C(29), C(30), C(38), C(39) and C(40)] in 4 showed high
thermal parameters and were refined isotropically with
shared occupancy factors.
The conformational behavior of ‘‘P–N–P’’ type ligands
is well known in the literature [14c,21]. It has been found
that the preferred conformation (see Chart 1) depends on
Table 5
Summary of X-ray diffraction data for ligand L1 and the complexes 4, 7 and 18
L1
4 Æ CH3CH2OH
7
18
Empirical formula
Formula weight
Crystal system, space
group
C
575.59
Monoclinic, P2/n
33H39NO4P2
C55H66NO5P2ClRu
1019.55
C38H45NO4P2Ru
742.76921.76
Monoclinic, C2/c
C43H49N3O4P2Cl2Ru Æ H2O
ꢀ
Triclinic, P1
Orthorhombic, P21cn
˚
˚
˚
˚
˚
˚
˚
Unit cell dimensions
a = 12.045(7) A
b = 10.263(6) A
a = 11.642(2) A
a = 43.0433(14) A
a = 11.050(5) A
˚
b = 14.017(3) A
b = 9.5072(3) A
b = 13.017(7) A
˚
˚
˚
˚
c = 12.478(7) A
c = 16.710(3) A
c = 19.0176(7) A
c = 31.657(15) A
b = 99.117(9)ꢁ
a = 77.173(4)ꢁ
b = 84.107(3)ꢁ
c = 86.093(4)ꢁ
2641.8(9)
2
b = 111.414(2)ꢁ
3
˚
Volume (A )
Z
1523.0(14)
2
7245.2(4)
8
4554(4)
4
Density (calcd), Mg/
mm3
1.255
1.282
1.362
1.344
Absorption coefficient
0.180
0.454
0.560
–
0.576
–
(mmꢀ1
)
Max. and min.
transmission
F(000)
0.958 and 0.8581
0.9772 and 0.8049
612
1068
3088
1904
Crystal size (mm)
h Range for data
collection
0.875 · 0.641 · 0.240
1.98–27.0ꢁ
0.500 · 0.240 · 0.050
1.73–27.5ꢁ
0.200 · 0.150 · 0.100
1.02–23.25ꢁ
0.195 · 0.049 · 0.019
1.69–28.24ꢁ
Index ranges
ꢀ15 6 h 6 15, ꢀ13 6 k 6 13, ꢀ15 6 h 6 15, ꢀ18 6 k 6 18, ꢀ47 6 h 6 47, ꢀ9 6 k 6 10,
ꢀ14 6 h 6 13, ꢀ17 6 k 6 17,
ꢀ41 6 l 6 40
ꢀ15 6 l 6 15
ꢀ21 6 l 6 21
ꢀ21 6 l 6 19
Reflections collected
15,227
31,013
14,453
38,288
Independent reflections 3332 [R(int) = 0.0745]
12,200 [R(int) = 0.0439]
Full-matrix least-squares on
F2
5209 [R(int) = 0.0645]
Full-matrix least-squares on Full-matrix least-squares on
F2
10,279 [R(int) = 0.1433]
Refinement method
Full-matrix least-squares on
F2
F2
Data/restraints/
parameters
Goodness-of-fit on F2
Final R indices
[I > 2r(I)]
3332/0/186
12,200/0/590
5209/0/419
10,279/1/500
1.249
1.069
0.999
0.950
R1 = 0.048, wR2 = 0.1263
R1 = 0.0710, wR2 = 0.1666
R1 = 0.0469, wR2 = 0.0902
R1 = 0.0970, wR2 = 0.1805
R indices all data
Absolute structure
parameter
R1 = 0.0613, wR2 = 0.1329
0.473 and ꢀ0.291
R1 = 0.1056, wR2 = 0.1829
0.738 and ꢀ0.375
R1 = 0.0895, wR2 = 0.1048
0.408 and ꢀ0.436
R1 = 0.2540, wR2 = 0.3027
0.02(9)
Largest diff ꢀp3eak and
1.946 and ꢀ0.658
˚
hole (e A
)

T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
1881
nium complexes of chiral diphosphazane ligands based
on the P–N–P motif [12a,25].
R
N
R
N
R
N
X
.
.
X
X
X
.
. .P
.
X
The reaction of [CpRu(PPh3)2Cl] with one equivalent of
L1 in boiling benzene affords the diphosphazane substi-
X
P
P.
P
P.
..P
.
.
X
X
X
X
[(g5-Cp)RuCl{j2-P,P-(RO)2PN(Me)-
X
X
tuted
complex
IA
IB
IC
C2V
P(OR)2}] (1) (Scheme 1). Complexes 2–4 are obtained by
the reduction of [Cp*RuCl2]n using zinc dust in the pres-
ence of diphosphazanes L2–L4. When such a reduction is
carried out in the presence of the unsymmetrically substi-
CS
C2V
'
Chart 1. Conformations of diphosphazanes.
tuted
diphosphazanes
(rac)-(C20H12O2)PN(CHMe2)-
the substituents present on the nitrogen and the phospho-
rus centers. In general, when the substituents R and X
are relatively small, a conformation of type 1B is preferred.
But, when the substituents R and/or X are large, a confor-
mation of type 1A is preferred [21b]. An X-ray crystallo-
graphic study (see below) reveals that L1 adopts a
conformation of type 1B (C2 conformer) in the solid state
as observed recently for the sterically bulky diphospha-
P(OR)2 [R = C6H5 (L3) or C6H4 Bu-4 (L4)], the product
would possess a stereogenic metal center. Since the binaph-
thyl moiety is racemic and one sterogenic center is created
during the course of the reaction, there exists the possibility
of the formation of four diastereomers [(RaRRu), (SaSRu),
(RaSRu) and (SaRRu)] (two enantiomeric pairs). Hence
one would expect the 31P NMR spectrum to display two
AX spin systems (four doublets) for the two enantiomeric
pairs. The 31P NMR spectrum of the reaction mixture dis-
plays only two doublets at 124.2 and 107.3 ppm
[2J(P,P) = 61.2 Hz] for the P(O2C20H12) and P(OR)2 phos-
phorus nuclei respectively, in addition to the resonances
corresponding to those of the starting diphosphazane indi-
cating stereoselective formation of only one enantiomeric
pair. Faller and Parr have observed a similar diasteroselec-
tivity in the formation of (arene)ruthenium complexes
of 2,20-bis(diphenylphosphino)-1,10-binaphthyl monoxide
[24n]. Complexes 1–4 have been characterized by NMR
spectroscopic techniques and the data are listed in Table
2. The 31P chemical shifts of complexes 1–4 are upfield
shifted from that of the free ligand (Dd = ꢀ26.3 to
ꢀ36.4). The structure of complex 4 has been established
by single crystal X-ray crystallography. The solid-state
structure (see later) reveals the formation of only one enan-
tiomeric pair [(RaRRu) and (SaSRu)].
t
i
zanes, EtN{P(OR)2}2 (R = C6H3 Pr2-2,6 or C6H3Me2-2,6)
[21d]. A variable temperature NMR spectrum of the ligand
L1 in the range ꢀ40 to +55 ꢁC (CDCl3) does not show any
change in the spectral pattern indicating that in this tem-
perature range, only one conformer is present in solution
owing to the increased rigidity of the ligand.
3.2. Synthesis of half-sandwich (Cp and Cp*) ruthenium
chloro complexes of diphosphazanes
The synthesis of half-sandwich optically active ‘‘chiral-
at-metal’’ ruthenium complexes has received considerable
interest in recent years because they can be used as probes
to follow the stereochemical course of reactions of organo-
metallic complexes and they have also been investigated as
promising catalysts for a variety of organic transforma-
tions [22]. Active use of chirality at a metal center in enan-
tioselective catalysis is a challenging area [22c,23]. Several
ruthenium complexes with a stereogenic metal center have
been synthesized using Schiff-base ligands, chiral bidentate
ligands with mixed donor sites or chiral diphosphanes that
bear chirality at the back-bone of the ligand [24]. There are
only two reports on the synthesis of half-sandwich ruthe-
3.3. Synthesis of half-sandwich (Cp and Cp*) ruthenium
hydride complexes of diphosphazanes
Transition metal hydride complexes and their dihydro-
gen analogues are of great importance in view of the
L
R
Ru
Cl
Y
N
X
X
(i)
X2P
P
P
PYY'
Y'
N
R
R = Me, X = Y = Y' = OC6H3Me2-2,6 (L1)
R = Me, X = Y = Y' = OC6H5 (L2)
L = Cp, R = Me, X = Y = Y' = OC6H3Me2-2,6 (1)
L = Cp*, R = Me, X = Y = Y' = OC6H5 (2)
L = Cp*, R = CHMe2, X2 = O2C20H12, Y = Y' = OC6H5 (3)
R = CHMe2, X2 = O2C20H12, Y = Y' = OC6H5 (L3)
R = CHMe2, X2 = O2C20H12, Y = Y' = OC6H4 Bu-4 (L4)
L = Cp*, R = CHMe2, X2 = O2C20H12, Y = Y' = OC6H4 Bu-4 (4)
t
t
(i) CpRu(PPh3)2Cl, benzene, 80 0C, 60h (for 1)
[Cp*RuCl2]n, toluene, zinc dust, 25 0C, 24h (for 2-4)
Scheme 1. Synthesis of half-sandwich (Cp and Cp*) ruthenium chloro complexes of diphosphazanes.

1882
T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
presence of such species as intermediates in several catalytic
reactions. There are numerous reports on transition metal
(Fe, Ru, Os, W) hydride complexes bearing mono- and
diphosphines [26–28] but similar studies using diphospha-
zanes as ancillary ligands are very much limited [25,29].
The cyclopentadienyl ruthenium chloro 0 complexes
[CpRuCl{j2-P,P-Ph2PN((S)-*CHMePh)PPh(Y )}] [Y0 =
Ph (5), N2C3HMe2-3,5 (6)] were prepared from the reaction
(SCSPRRu) and (SCSPSrRu) for the two diastereomers
assuming that the phosphorus chirality is unaffected during
the reaction. Recrystallization of the mixture from deoxy-
genated n-pentane yields the major diastereomer as a pale
yellow solid. The NMR data for these complexes are listed
in Table 2.
3.4. Protonation of the ruthenium hydride complex,
[CpRuH{j2-P,P-(RO)2PN(Me)P(OR)2}]
(R = C6H3Me2-2,6) (7)
between
[(g5-Cp)Ru(PPh3)2Cl]
and
Ph2PN(R)P-
Ph(Y0) [Y0 = Ph (L5), N2C3HMe2-3,5 (L9)] as reported
earlier [12a]. The reactions of the half-sandwich ruthenium
chloro complexes (1, 2, 4–6) with NaOMe in methanolic
solutions result in the formation of the ruthenium hydride
complexes [CpRuH{j2-P,P-X2PN(R)PY(Y0)}] (7–11) in
good yields (Scheme 2). These complexes have been charac-
terized mainly by NMR spectroscopic technique (Table 2).
The 31P NMR spectrum of complex 7 displays a singlet at
120.6 ppm that is upfield shifted from that of the free ligand
(Dd = ꢀ14.5). A similar upfield shift in the 31P resonance is
Though there are several reports on the protonation of
half-sandwich ruthenium hydride complexes bearing
mono- and diphosphines [26,27], there are no reports on
the protonation of ruthenium hydride complexes bearing
‘‘P–N–P’’ type ligands. Protonation of CpRu(P–P)H com-
plexes (P–P = dppm, dppe or dppp) yield exclusively dihy-
drogen [for dppm] or dihydride complexes [for dppp] or a
mixture of both [for dppe] [27a]. It is reported that proton-
ation of Cp*RuH(L2) [L2 = dppm, dppp, (PPh3)2,
(PMe2Ph)2, (PMePh2)2, (PMe3)2] with HBF4 Æ Et2O gives
a mixture of dihydrogen and dihydride species, the compo-
sition of which depends on the nature of the phosphine
bound to ruthenium. With the exception of dppm, the ther-
modynamic product was found to be the dihydride com-
plex. In the case of dppm, the final protonation product
was a 2:1 mixture of dihydrogen and dihydride complexes.
Such a difference in the Cp and Cp* analogues was attrib-
uted to the increase in electron density at ruthenium on
going from Cp to Cp* ligand [27d]. The analogous [(g5-
C9H7)Ru(H)(P–P)] [27g] complex exhibits a reactivity pat-
tern similar to the [CpRu(H)(P–P)] complex. According to
Heinekey and co-workers [27b], the initial kinetic product
is always the dihydrogen complex. So far, it has been diffi-
cult to determine if the dihydride complex is obtained by
direct protonation at the ruthenium or by oxidative addi-
tion of the dihydrogen complex. The remarkable stability
of the ruthenium hydride complex 7 provided us the oppor-
tunity to investigate its reactivity towards different acids
(CF3SO3H, HBF4, and CF3COOH) to ascertain which of
the two forms, dihydrogen or dihydride, would be stabi-
1
observed for complex 8 (Dd = ꢀ21.4). The H NMR spec-
trum of 7 shows a triplet at ꢀ13.06 ppm for the ruthenium
bound hydride hydrogen. For complex 8, the Ru–H reso-
nance is observed at ꢀ11.99 ppm. While complex 7 was iso-
lated in pure form and its solid-state structure established
by single crystal X-ray crystallography, complexes 8, 9
and 10 could not be isolated in pure form owing to their
high sensitivity to air and chlorinated solvents. Gamasa
and co-workers [25] have reported the synthesis and struc-
tural characterization of the indenyl analogue of complex
10, [(g5-C9H7) Ru(H){j2-P,P-Ph2PN((S)-*CHMePh)-
PPh2}] from the corresponding chloro complex.
The reaction involving the trichiral half-sandwich com-
plex [CpRuCl{j2-P,P-Ph2PN((S)-CHMePh)PPh(N2C3H-
Me2-3,5)}] (SCSPRRu)-6 results in the formation of
a diastereomeric mixture of hydride complexes (11a
and 11 b) as revealed by NMR spectroscopy. The 31P
NMR spectrum shows two AX patterns corresponding
to 11a and 11b. The 1H NMR spectrum displays two
doublet-of-doublets at ꢀ11.06 (major) and ꢀ11.86 ppm
(minor) for the hydride hydrogen corresponding to the
two diastereomers. We tentatively assign the configurations
L
L
Ru
Cl
Ru
H
(i)
X2P
X2P
PYY'
PYY'
N
R
N
R
L = Cp, R = Me, X = Y = Y' = OC6H3Me2-2,6 (1)
L = Cp*, R = Me, X = Y = Y' = OC6H5 (2)
L = Cp*, R = CHMe2, X2 = O2C20H12, Y = Y' = OC6H4 Bu-4 (4)
L = Cp, R = (S)-*CHMePh, X = Y = Y' = Ph (5)
L = Cp, R = (S)-*CHMePh, X = Y = Ph, Y' = N2C3HMe2-3,5 (6)
L = Cp, R = Me, X = Y = Y' = OC6H3Me2-2,6 (7)
L = Cp*, R = Me, X = Y = Y' = OC6H5 (8)
L = Cp*, R = CHMe2, X2 = O2C20H12, Y = Y' = OC6H4 Bu-4 (9)
L = Cp, R = (S)-*CHMePh, X = Y = Y' = Ph (10)
L = Cp, R = (S)-*CHMePh, X = Y = Ph, Y' = N2C3HMe2-3,5 (11)
t
t
(i) CH3ONa, MeOH, 60 0C, 20h
Scheme 2. Synthesis of half-sandwich (Cp and Cp*) ruthenium hydrido complexes of diphosphazanes.

T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
1883
lised when a strong p-acceptor ligand is bound to ruthe-
nium. The protonation experiments were carried out in a
sealed NMR tube containing a CDCl3 solution of complex
7 at ambient temperature.
Measurement of relaxation time T1 was carried out at
298 K and 400 MHz to ascertain the nature of the hydride
in complexes 12a and 12b. The broad resonance at
ꢀ8.00 ppm has a T1 value of 10 ms. The triplet at
ꢀ6.25 ppm has a T1 value between 150–200 ms. These val-
ues indicate the absence of any intramolecular exchange
process [27a]. In order to further confirm the dihydrogen
nature of the resonance at ꢀ8.00 ppm, HD gas (generated
from D2O and NaH) was purged into the reaction mixture
to generate the H–D isotopomer of 12a. No significant
quantity of the expected H–D isotopomer was formed (as
Treatment of complex 7 with an excess of HX
(X = CF3SO3, BF4 or CF3COO) results in the formation
of a mixture of two complexes, [CpRu(g2-H2){j2-P,
P-(RO)2PN(Me)P(OR)2}][CF3SO3] (12a) and [CpRu(H)2-
{j2-P,P-(RO)2PN(Me)P(OR)2}][CF3SO3] (12b) (Scheme
3). Depending on the acid used for protonation, the ratio
between 12a and 12b varies. Nevertheless, the major prod-
uct in all the cases is the dihydrogen complex 12a presum-
ably because of the steric bulk of the diphosphazane ligand.
1
shown by the absence of a 1:1:1 triplet in the H NMR
spectrum).
1
The H NMR spectrum of a CDCl3 solution of 7 upon
In order to prepare the H–D isotopomer, we synthesized
the deuteride complex 7-D from CD3OD (similar to 7 as
shown in Scheme 2). The 31P NMR spectrum of 7-D displays
a triplet at 120.7 ppm. Protonation of 7-D using HOTf
affords a mixture of [CpRu(g2-HD){j2-P,P-(RO)2PN(Me)-
P(OR)2}][CF3SO3] (13a) and [CpRu(H)(D){j2-P,P-
(RO)2PN(Me)P(OR)2}][CF3SO3] (13b). A triplet is observed
at ꢀ8.01 ppm [1J(H,D) = 24.5 Hz] for the side-on g2 bound
H–D isotopomer in 13a. A triplet-of-triplets pattern is
observed for the hydride proton of 13b at ꢀ6.20 ppm
[2J(P,H) = 26.4 Hz, 2J(H,D) = 4.0 Hz]. The 31P NMR spec-
trum displays a triplet at 115.6 ppm [2J(P,D) = 13.8 Hz] and
a broad resonance at 108.8 ppm for 13a and 13b respectively.
Having synthesized the dihydrogen complex and the
corresponding H–D isotopomer, we estimated the H–H
distance from the T1 values [28a,b] and also from the
1J(H,D) values [28c]. The T1 value of 10 ms corresponds
the addition of HX displays a triplet atꢀ6.20 ppm and a
broad resonance atꢀ8.00 ppm. The former resonance is
assigned to dihydride hydrogens in the Ru(IV) complex
12b and the latter to the bound dihydrogen in the Ru(II)
complex 12a. The cyclopentadienyl hydrogens in com-
plexes 12a and 12b resonate at 4.65 and 4.71 ppm respec-
tively. No separate resonances were observed for the
methyl protons bound to nitrogen and those at the ortho
positions of the phenoxy substituents in complexes 12a
and 12b. The 31P NMR spectrum displays two singlets at
115.5 and 107.8 ppm which are assigned to 12a and 12b
respectively.
Variable temperature (VT) NMR spectrum (ꢀ40 to
+50 ꢁC, CDCl3) shows a variation in the ratio of the two
species as a function of temperature, but does not show
complete coalescence of the resonances in the temperature
limit studied. The VT NMR spectrum reveals that complex
12a is the dominant species throughout the temperature
range ꢀ40 to +50 ꢁC. The proportion of 12b increases as
the temperature is increased from ꢀ40 ꢁC, reaches a maxi-
mum at +20 ꢁC (1:8) and then starts dropping again with a
further increase in temperature.
˚
to a H–H distance (rH–H) of 0.788 A if the H–H rotation
is faster on the NMR time scale. If the H–H rotation is
slower on the NMR time scale, then the H–H distance is
1
˚
0.944 A. The medium value of J(H,D) coupling constant
for the H–D isotopomer (24.5 Hz) gives a H–H distance
˚
of 1.011 A. This value is close to the value observed for
+
+
-
-
Cp
Ru
Cp
Cp
Ru
X
X
X
12a 12b
H
H
H
H
Ru
H
HX
+
CF3SO3
BF4
CF3COO 12
8
13
1
1
1
(RO)2P
Me
(RO)2P
Me
(RO)2P
Me
P(OR)2
P(OR)2
P(OR)2
N
N
N
OR = OC6H3Me2-2,6 (7)
12a
12b
+
+
-
-
Cp
Ru
Cp
Cp
CF3SO3
CF3SO3
H
D
H
D
Ru
D
Ru
CF3SO3H
+
(RO)2P
Me
(RO)2P
(RO)2P
P(OR)2
P(OR)2
P(OR)2
N
N
N
Me
Me
OR = OC6H3Me2-2,6 (7-D)
13a
13b
Scheme 3. Protonation of complex 7 using different acids.

1884
T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
the complexes [(g5-C9H7)Ru(H2)(dppm)] [27g] and [CpRu-
(H2)(dppm)] [28d]. Thus we conclude that the dihydrogen
ligand in our case is in the slow rotating regime with a
*
CHMePh
CHMe2
CH3
N
N
N
P
OR
OR
O
O
O
O
O
O
P
Ph2P
PPh2
P
P
˚
H–H bond distance of 0.944 A.
OR = OC6H5; (S)-L3
L5
(R, R)-L7
(S, S)-L7
OR = OC6H4 Bu-4; (R)-L4
t
3.5. Catalytic transfer hydrogenation of ketones
t
OR = OC6H4 Bu-4; (S)-L4
*CHMePh
CHMe2
The access to a range of symmetrically and unsymmetri-
cally substituted diphosphazanes that differ in steric bulk
and p-acceptor capability prompted us to investigate their
catalytic activity in asymmetric transfer hydrogenation of
ketones. A plethora of ruthenium complexes are known
for this transformation [2–11]. No chiral diphosphazane
based ruthenium catalysts have been used to-date for this
transformation. Asymmetric transfer hydrogenation of 2-
acetonaphthone with 2-propanol was used as the model
reaction (Scheme 4). In a typical experiment, the catalyst
was generated in situ from [RuCl2(PPh3)3] and two equiva-
N
Ph
N
O
O
Ph2P
P
PPh2
P
*
O
O
N
N
=
O
O
(R)- L8
(S)- L8
(SCRP)-L9
Chart 2. Chiral diphosphazanes employed in asymmetric transfer
hydrogenation.
3.6. Synthesis of ruthenium(II) chloro(hydrido) complexes
of diphosphazanes
lents of chiral diphosphazane and
a ketone:ruthe-
It is well known that ruthenium catalyzed asymmetric
transfer hydrogenation proceeds through a hydride inter-
mediate. In order to have an insight into the nature of
the hydride species generated under catalytic conditions,
we attempted to synthesize chloro and hydrido complexes
of ruthenium. Recently, trans-dichloro [1i,6b,8d,11b] and
trans-(chloro)hydrido [1i,6b,11a,30] complexes of ruthe-
nium bearing diphosphines have attracted attention
because of their potential usefulness as catalysts in hydro-
genation and transfer hydrogenation; some of the com-
plexes have been structurally characterized. The synthesis
and structural characterization of trans-dichloro ruthenium
complexes of diphosphazanes have also been reported [31].
The reaction of [RuCl2(COD)]n with two molar equiva-
lents of Ph2PN(R)PPh2 [R = (S)-*CHMePh (L5) or
CHMe2 (L6)] in the presence of an excess of triethylamine
in ethanol give the trans chloro hydrido ruthenium com-
plexes trans-[Ru(H)Cl{j2-P,P-Ph2PN(R)PPh2}2] [R = (S)-
*CHMePh (14) or CHMe2 (15)] (Scheme 5). The 31P
NMR spectrum of 14 displays an AA0XX0 pattern for
the diastereotopic phosphorus atoms. A singlet is observed
at 87.5 ppm for complex 15. The 1H NMR spectra of com-
plexes 14 and 15 display a quintet pattern at ꢀ15.20 ppm
for the hydride ligand trans to the chloride. The chemical
shift, multiplicity and coupling constants reveal that the
hydride is located transoid to chloride and cisoid to two
diphosphazane ligands. Based on the observed spectro-
scopic pattern, we conclude that 14 and 15 are the
bis(diphosphazane) chelated Ru(II) derivatives, trans-
[Ru(H)Cl{j2-P,P-Ph2PN(R)PPh2}2], in which the hydride
ligand is located trans to the chloride. The observed
hydride chemical shift is close to that reported for other
similar type of complexes [30].
nium:ligand ratio of 200:1:2 was used. Chart 2 shows the
various chiral diphosphazanes that have been used for
transfer hydrogenation and the results are shown in Table
3. It is evident that the ligands (R,R)-L7, (S,S)-L7, (R)-L4
and (S)-L4 induce moderate levels of enantioselectivity in
the product while the other ligands do not impart any
enantioselectivity. Apparently, axially chiral diphospha-
zanes that bear sterically encumbered substituent at the
phosphorus are more efficient in chiral induction than the
other ligands, though the yield of the product is low.
Ligands L5 and L9 that bear chirality at the carbon are less
sterically encumbered at the phosphorus and do not give
rise to enantioselectivity. Though ligands L3 and L8 bear
the axially chiral 1,10-binaphthylene-2,20-dioxy moiety,
one of the phosphorus centers in these ligands is not steri-
cally crowded; hence these ligands do not impart enantiose-
lectivity. The formation of a diastereomeric mixture of
ruthenium hydride complexes 11a and 11b from the opti-
cally pure trichiral complex 6 and the stereoselective forma-
tion of only one enantiomeric pair during the synthesis of
complex 4 clearly bring out the importance of steric effect
in controlling the enantioselectivity. In an attempt to
enhance the enantioselectivity and also to investigate the
influence of catalyst ratio on enantioselectivity, the ratio
of ketone:ruthenium:ligand was decreased to 100:1:2. The
axially chiral ligand (R,R)-L7 was used for this study. Upon
increasing the mole ratio of the catalyst from 0.5 to 1.0, we
found a moderate increase in the enantioselectiviy of the
product from 10% to 30% (see Table 3).
O
OH
OH
(i)
+
In contrast to the reactions of the ligands L5 and L6
towards [RuCl2(COD)]n, the reaction of [RuCl2(COD)]n
with either one or two molar equivalents of the sterically
bulky diphosphazane (RO)2PN(Me)P(OR)2 [R = C6H3-
Me2-2,6 (L1)] under similar conditions gives a product,
(i) RuCl2(PPh3)3, 2 equiv. ligand, 3 equiv. KOH, 80 0C
Scheme 4. Ruthenium catalyzed asymmetric transfer hydrogenation of 2-
acetonaphthone.

T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
1885
X
X
P2
P2
H
+
-
Ph2
P
Ph2
P
CF3SO3
H
N
R
R
N
Ru
(v)
P
X2
P
N
R
R
N
Ru
Cl X2
P
Ph2
P
X = Ph, R = (S)-*CHMePh (14)
X = Ph, R = CHMe2 (15)
PR'3 Ph2
R = (S)-*CHMePh
R' = OMe (19)
R' = OPh (20)
(i)
X
P2
H
S
R
N
R
N
Ru
(ii)
PX2
X2P
P
X2
S
Cl
R = Me,
X = OC6H3Me2-2,6 (16)
(iii)
R
R
N
(RO)2P
Cl
P(OR)2
P(OR)2
N
N
N
(iv)
(RO)2P
Ru
Cl
Ru
N
H
N
Cl
R = Me, OR = OC6H3Me2-2,6 (17)
R = Me, OR = OC6H3Me2-2,6 (18)
(i) 0.5 eq. [RuCl2(1,5-COD)]n, Et3N, EtOH, 80 0C, 5 h
(ii) 0.5 or 1 eq. [RuCl2(1,5-COD)]n, Et3N, EtOH, 80 0C, 5 h
(iii) 2,2'-bipyridine, CH3CN + THF (5:2 v/v), 25 0C, 75 mins
(iv) Crystallization from dichloromethane-petrol
(v) PR'3, AgOTf, THF, 25 0C, 3h
Scheme 5. Synthesis of ruthenium hydrido and chloro-hydrido complexes of diphosphazanes.
which can be formulated as the hydride complex, trans-
[RuH(Cl){j2-P,P-(RO)2PN(Me)P(OR)2}(CD3CN)2] (16-d3)
ligand. Treatment of an acetonitrile–THF solution of com-
plex 16 with one equivalent of 2,20-bipyridine gives a ‘‘dark
red complex’’ (17). The 31P NMR spectrum of the ‘‘dark
red complex’’ shows an AB spin system at 108.9 and
107.7 ppm [2J(A,B) = 36.7 Hz] indicating the presence of
two magnetically non-equivalent phosphorus nuclei. The
1H NMR spectrum displays a ‘‘doublet-of-doublets’’ at
ꢀ13.39 ppm [2J(P,H) = 44.0, 33.3 Hz] for the ruthenium
bound hydride. Crystallization of the ‘‘dark red complex’’
from dichloromethane–petrol afforded bright yellow-
orange crystals. The 31P NMR spectrum of the crystals
shows an AB spin system at 97.6 and 91.8 ppm
[2J(A,B) = 9.7 Hz]. No hydride resonances were observed
in the 1H NMR spectrum. A single crystal X-ray crystallo-
graphic study (see later) was performed on the bright
yellow-orange crystals and the compound was found to
be cis-[RuCl2{j2-P,P-(RO)2PN(Me)P(OR)2}(j2-N,N-2,20-
bpy)] (bpy = 2,20-bipyridine) (18) (see Fig. 5). Since the
complexes 17 and 18 display an AB pattern in their 31P
NMR spectra, complex 17 is formulated as cis-[Ru(H)Cl-
{j2-P,P-(RO)2PN(Me)P(OR)2}(j2-N,N-bpy)] (17) in which
the hydride ligand is located trans to a bipyridyl nitrogen
atom. Apparently, complex 17 is obtained from complex
16 by trans–cis isomerization.
1
on the basis of its 31P and H NMR spectral data (Table
4). The phosphorus chemical shift for 16 (114.1 ppm) is
shifted upfield as compared to that of the parent diphosp-
hazane. The 1H NMR spectrum displays a triplet at
ꢀ14.88 ppm; this chemical shift value is close to that
observed for complexes 14 and 15 and indicates that the
hydride is located trans to a strongly basic ligand such as
chloride. The multiplicity of the hydride resonance reveals
that only one diphosphazane (two phosphorus nuclei) is
bound to ruthenium. Complex 16 could not be isolated in
a pure form owing to its high sensitivity to air and chlori-
nated solvents. The formation of only the mono diphosp-
hazane substituted complex 16 even in the presence of two
equivalents of the diphosphazane can be attributed to the
steric crowding provided by the methyl groups at the two
ortho positions of the phenoxy substituent, which precludes
the coordination of two diphosphazane ligands to
ruthenium.
Since complex 16 could not be isolated in pure form,
attempts were made to isolate a hydride complex bearing
one diphosphazane moiety by substitution of the labile
coordinated solvent molecules with a suitable bidentate

1886
T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
Recently, Albertin and co-workers [32] have synthesized
a series of hydride complexes, [M(H)(N–N)(L)3] (M = Fe
[32a], Ru [32b] or Os [32c]; L = monophosphorus ligand,
N–N = 2,20-bipyridine, 1,10-phenanthroline or 5,50-Me2-
2,20bipyridine) which display a spectroscopic behavior sim-
ilar to 17. These complexes were found to be fluxional in
solution and exist as a mixture of fac- and mer-isomers in
solution. The hydride resonance in these mer-isomers
appears in the range ꢀ11.9 to ꢀ18.4 ppm depending on
the transition metal and the phosphorus ligand trans to
the hydride ligand. The structure of the mer-isomer, cis-
[Ru(H){P(OEt)3}3(j2-N,N-bpy)] was established by X-ray
crystallography [32b].
3.7. Substitution reactions on complex 14
The reaction of complex 14 with monophosphites
P(OR)3 (R = Me or Ph) in the presence of AgOTf afford
the phosphite substituted cationic ruthenium complexes
Fig. 1. ORTEP view of the molecular structure of (RO)2PN(Me)P(OR)2
(L1). Thermal ellipsoids are drawn at 30% probability level. Hydrogen
atoms are omitted for clarity.
1
19 and 20. The H NMR spectrum of 19 or 20 displays a
doublet-of-quintet pattern atꢀ6.20 ppm for the hydride
trans to the phosphite ligand. This chemical shift lies down-
field from that observed for complexes 14 or 15. The 31P
NMR spectrum displays an AX4 spin system. The phos-
phite phosphorus in complexes 19 or 20 appears at
ꢁ118–129 ppm as a quintet. The four phosphorus nuclei
of the two diphosphazane ligands resonate as a doublet
at ꢁ89 ppm. The NMR data for the complexes 14–20 are
listed in Table 4. The observed NMR spectral pattern for
complexes 19 or 20 is similar to that observed for the com-
plexes trans-[RuH{P(OR3)}(dppe)2] (R = Me, Et, iPr) [33].
Attempts to synthesize a dihydrogen complex from com-
plex 14 either by (i) protonation using different acids or
(ii) abstraction of chloride by AgOTf followed by reaction
with H2 gas were unsuccessful.
˚
(CHMe2)PPh(OC6H3Me2-2,6) (1.71 A) [34a] and close to
i
the values for EtN{P(OR)2}2 (R = C6H3 Pr2-2,6 or
C6H3Me2-2,6) (1.674 and 1.693 A) [21d]. The P–N dis-
tances in (C6H4O2)PN(R)PPh2 [34b] that bears two differ-
ent type of phosphorus atoms shows two different P–N
bond lengths. The phosphorus that is less p-acceptor in
character (‘‘PPh2’’ phosphorus) is farther from the nitrogen
(1.743A) than the phosphorus that is a stronger p-acceptor
(1.654 A). These variations in P–N bond distances can be
rationalized on the basis of ‘‘negative hyperconjugation’’
model [35].
˚
˚
˚
3.9. Crystal and molecular structures of the half-sandwich
complexes 4 and 7
The solid-state structures of the complexes 4 and 7 are
shown in Figs. 2 and 3 respectively. A comparison of the
steric influence of the ligands in complexes 4 and 7 is
depicted in Fig. 4. The structures belong to the class of
3.8. Crystal and molecular structure of
(RO)2PN(Me)P(OR)2 (L1)
The structure of ligand L1 is shown in Fig. 1. The
compound crystallizes in the space group P2/n with half
a molecule in the asymmetric unit and two molecules in
the unit cell. The whole molecule adopts a C2 conformation
in the solid-state. The geometry around the nitrogen is
trigonal planar with a P–N–P angle of 115.3(1)ꢁ. This value
is smaller than that in Ph2PN(CHMe2)PPh2 (122.8ꢁ) [21b],
Ph2PN(CHMe2)PPh(OC6H3Me2-2,6) (122.0ꢁ) [34a] (C6H4-
i
O2)PN(R)PPh2 (120.7ꢁ) [34b] and PrN[PhPX][PhPX0] (X,
X0 = iPrNH, EtNHt or PhNH) (119.6–120.7ꢁ) [21c] but
slightly larger than that observed in Ph2PN(R)PPh2
[R = C6H5, C6H4(CN-3), C6H4(CN-4)] and EtN{P(OR)2}2
i
(R = C6H3 Pr2-2,6 or C6H3Me2-2,6) (109.5–114.5ꢁ)
[21d,34c]. The P–N bond distance in L1 [1.672(1) A] is
˚
shorter than that in Ph2PN(R)PPh2 [R = CHMe2
˚
(1.710 A) [21b], C6H5, C6H4(CN-3) or C6H4(CN-4)
Fig. 2. ORTEP view of the molecular structure of [(g5-Cp*)Ru{j2-P,P-
(C20H12O2)PN(CHMe2)P(OC6H4 Bu-4)2}Cl] (4). Thermal ellipsoids are
i
i
0
0
˚
t
(1.723–1.746 A) [34c], Pr[PhPX][PhPX ] (X, X = PrNH,
EtNH or PhNH) (1.705–1.722 A) [21c], and Ph2PN-
˚
drawn at 30% probability level. Hydrogen atoms are omitted for clarity.

T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
1887
three-legged piano-stool complexes; the two phosphorus
atoms of the diphosphazane and the hydride (or chloride)
ligands act as the legs. The coordination geometry around
ruthenium can be considered as distorted octahedral.
Selected bond distances and angles are listed in Table 6.
The bite angle at Ru(1) in complexes 4 and 7 [67.77(4)ꢁ
and 69.21(5)ꢁ respectively] are comparable to each other
and similar to that observed in the trichiral cyclopentadie-
nyl ruthenium complex (SCSPRRu)-[(g5-C5H5)Ru{j2-P,
P-Ph2PN(R)PPh(N2C3HMe2-3,5)}Cl] (6) [R = (S)-*CH-
MePh] [69.3(1)ꢁ] [12a] and the indenyl ruthenium hydride
complex [(g5-C9H7)Ru(H){j2-P,P-Ph2PN((S)-*CHMePh)-
PPh2}] [71.5(1)ꢁ] [25]. The P–N–P span angle in complexes
4 and 7 [99.4(2)ꢁ and 95.8(2)ꢁ] are much smaller than that
in complex 6 [119.4(8)ꢁ]. This lowering of the span angle of
the diphosphazane in these two complexes is due to the
increased steric bulk at the phosphorus center. The pen-
tamethyl cyclopentadienyl ring deviates slightly from its
ideal planar geometry. In both the complexes, the monod-
entate ligand (chloride or hydride) is located perpendicular
to the diphosphazane. The angles subtended by the cen-
troid of the cyclopentadienyl moiety with the two phospho-
rus atoms of diphosphazane in complex 7 are almost equal
(141.7ꢁ and 142.6ꢁ); the corresponding values for 4 are
133.5ꢁ and 140.1ꢁ. Because of the asymmetric nature of
the diphosphazane in complex 4, the pentamethyl cyclo-
pentadienyl ring is tilted slightly towards the binaphthyl
phosphorus P(1) [hence a smaller angle, 133.5ꢁ for Cp–
Ru–P(1)]; the bulky tert-butylphenoxy substituents on the
phosphorus force the pentamethyl cyclopentadienyl moiety
towards the other side of the diphosphazane. A similar tilt-
ing of Cp* ring has been observed for the cationic complex,
[(g5-Cp*)Ru(g2-H2)dppm] BF4 complex [27d]. The solid-
state structure of complex 4 reveals that it consists of the
(RaRRu) and (SaSRu) enantiomeric pair. Such a stereoselec-
tive formation of only one enantiomeric pair can be attrib-
uted to the steric hinderance offered by the pentamethyl
cyclopentadienyl moiety to the bulky substituents at the
phosphorus of the diphosphazane ligand.
Fig. 3. ORTEP view of the molecular structure of [(g5-Cp)Ru(H){j2-P,P-
(RO)2PN(Me)P(OR)2}] (R = C6H3Me2-2,6) (7). Thermal ellipsoids are
drawn at 30% probability level. Hydrogen atoms (except ruthenium bound
hydride) are omitted for clarity.
3.10. Crystal and molecular structure of complex 18
Fig. 4. A view of the molecular structures of the half-sandwich complexes
4 and 7 depicting the steric influence of the ligands.
Thestructure ofthecomplexcis-[RuCl2{j2-P,P-(RO)2PN-
(Me)P(OR)2}{j2-N,N-2,20-bipyridine] (R = C6H3Me2-2,6)
Table 6
2
t
˚
Selected bond lengths (A) and bond angles (ꢁ) in half-sandwich complexes [Cp*RuCl{j -P,P-(C20H12O2)PN(CHMe2)P(OC6H4 Bu-4)2}] (4) and
[CpRuH{j2-P,P-(RO)2PN(Me)P(OR)2}] [R = C6H3Me2-2,6 (7)]
˚
Bond distances (A)
Bond angles (ꢁ)
4
7
4
7
Ru(1)–P(1)
Ru(1)–P(2)
Ru(2)–Cl(1)
Ru(1)–H(1)
Ru–CÆaveæ
P–NÆaveæ
2.201(2)
2.242(1)
2.425(1)
–
2.223(5)
1.688(3)
2.204(1)
2.197(1)
–
1.68(5)
2.251(6)
1.685(4)
P(1)–N(1)–P(2)
P(1)–N(1)–Calkyl
P(2)–N(1)–Calkyl
P(1)–Ru(1)–P(2)
P(1)–Ru(1)–Cpa
P(2)–Ru(1)–Cpa
95.8(2)
94.4(2)
131.8(3)
132.4(3)
69.21(5)
133.5
129.9(3)
133.7(3)
67.77(4)
141.7
140.1
142.6
a
Centroid of the cyclopentadienyl or pentamethyl cyclopentadienyl moiety.

1888
T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
a better p-acceptor than chloride ligand). The two P–N
bond distances are slightly different from each other
because the two phosphorus atoms are located at different
positions (axial and equatorial).
4. Conclusions
Half-sandwich (Cp and Cp*) ruthenium chloro com-
plexes of chiral and achiral diphosphazanes and their
hydride analogues have been synthesized. The reactions
of unsymmetrically substituted diphosphazanes bearing
axially chiral 1,10-binaphthyl-2,20-dioxy moiety with [(g5-
Cp*)RuCl2]n afford ‘‘chiral-at-metal’’ half-sandwich ruthe-
nium chloro complexes. The reaction proceeds by a path
in which the steric repulsion between the Cp* moiety and
the substituents on the phosphorus favors the formation
of only one enantiomeric pair. The stereochemistry at the
ruthenium in these complexes is determined by the configura-
tion of the binaphthyl moiety as established by X-ray crystal
Fig. 5. A view of the molecular structure of cis-[RuCl2{j2-P,P-
(RO)2PN(Me)P(OR)2}{j2-N,N-2,20-bipyridine] (R = C6H3Me2-2,6) (18).
Thermal ellipsoids are drawn at 30% probability level. Hydrogen atoms
are omitted for clarity.
structure
of
[(g5-Cp*)RuCl{j2-P,P-X2PN(R)PYY0}]
t
[R = CHMe2, X2 = C20H12O2, Y = Y0 = OC6H4 Bu-4]
(4). When the diphosphazane bound to ruthenium in the
(chloro) cyclopentadienyl complexes is chiral, the reaction
with NaOMe/MeOH to form the hydride derivative can
proceed stereoselectively if the substituents on the phospho-
rus are sterically bulky as in 4. Protonation of the ruthe-
nium hydride complex [CpRuH{j2-P,P-X2PN(Me)PX2}]
(X = OC6H3Me2-2,6) (7) affords mainly the dihydrogen
complex (12a) along with the dihydride complex (12b) in
smaller amounts owing to the steric crowding of the
diphosphazane around ruthenium. Strong p-acceptor char-
acter of the phosphorus center(s) and sterically bulky sub-
stituents attached to phosphorus appear to promote
enantioselectivity in Ru-catalyzed asymmetric transfer
hydrogenations using diphosphazanes as chiral auxiliaries.
The reaction of the sterically bulky ligand, X2PN(Me)PX2
(X = OC6H3Me2-2,6) (L1) with [RuCl2(COD)]n affords a
mono-diphosphazane substituted hydride complex (16);
such a mono-diphosphazane substituted complex could
be the intermediate in asymmetric transfer hydrogenation.
(18) is shown in Fig. 5. Since complex 18 is a dispropor-
tionated product and is obtained during crystallization of
complex 17, the quality of crystals and the diffraction data
were not very good. Hence, a detailed discussion of the
structural parameters is not attempted here. The complex
crystallizes in the chiral space group P21cn. Selected bond
distances and angles are listed in Table 7. The structure
reveals an octahedral ruthenium(II) complex in which
one of the phosphorus atoms of the diphosphazane ligand
is located at the equatorial position and the other is in the
axial position. The 2,20-bipyridine moiety and one chloride
ligand occupy the remaining coordination sites in the equa-
torial plane. The span angle of the diphosphazane
[100.6(5)ꢁ] is shorter than that of the free ligand. The phos-
phorus atom trans to the chloride ligand is slightly closer to
˚
ruthenium [Ru–P(2) = 2.229(3) A] than the phosphorus
that is located trans to the nitrogen atom of 2,20-bipyridine
˚
ligand [Ru–P(1) = [2.250(3) A] (bipyridyl nitrogen is a
better p-acceptor than chloride ligand). The chloride ligand
trans to the phosphorus is farther from ruthenium than the
other chloride ligand (phosphorus is a stronger p-acceptor
than nitrogen). The nitrogen atom of the bipyridine ligand
that is located trans to phosphorus is slightly farther from
5. Supplementary material
˚
ruthenium [Ru–N(3) = 2.129(9) A] than the other nitrogen
CCDC 628737, 628738, 628739, and 628740 contain the
supplementary crystallographic data for (L1), (4), (7), and
˚
atom of that ligand [Ru–N(2) = 2.09(1) A] (phosphorus is
Table 7
2
2
0
˚
Selected bond lengths (A) and bond angles (ꢁ) in the complex cis-[RuCl2{j -P,P-(RO)2PN(Me)P(OR)2}{j -N,N-2,2 -bipyridine] (R = C6H3Me2-2,6) (18)
˚
Bond distances (A)
Bond angles (ꢁ)
Ru(1)–P(1)
Ru(1)–P(2)
Ru(2)–Cl(1)
Ru(1)–Cl(2)
Ru(1)–N(2)
Ru(1)–N(3)
P(1)–N(1)
2.250(3)
2.229(3)
2.399(4)
2.479(4)
2.094(11)
2.129(9)
1.651(10)
1.691(9)
N(2)–Ru(1)–Cl(1)
N(2)–Ru(1)–Cl(2)
N(3)–Ru(1)–Cl(1)
N(3)–Ru(1)–Cl(2)
N(2)–Ru(1)–P(1)
N(2)–Ru(1)–P(2)
N(3)–Ru(1)–P(1)
N(3)–Ru(1)–P(2)
167.6(3)
83.0(3)
92.6(4)
83.8(3)
100.7(3)
96.8(3)
174.1(3)
104.6(3)
P(1)–N(1)–P(2)
100.6(5)
70.1(1)
90.2(1)
101.5(1)
92.4(1)
171.4(2)
77.2(5)
89.1(2)
P(1)–Ru(1)–P(2)
P(1)–Ru(1)–Cl(1)
P(1)–Ru(1)–Cl(2)
P(2)–Ru(1)–Cl(1)
P(2)–Ru(1)–Cl(2)
N(2)–Ru(1)–N(3)
Cl(1)–Ru(1)–Cl(2)
P(2)–N(1)

T.S. Venkatakrishnan et al. / Journal of Organometallic Chemistry 692 (2007) 1875–1891
1889
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